An orthogonal frequency division multiplexing (OFDM) scheme is a scheme for multi-carrier modulation. Use of the OFDM scheme enables sub-carrier overlapping using multiple sub-carriers which are orthogonal to each other. As a result, the use of the OFDM can raise efficiency in terms of transmitting a signal. Further, an OFDM system is a block unit-based transmission scheme being considered in preparation for a multi-path environment of a broadband wireless channel having a relatively long delay spread. Thus, an OFDM system enables equalization of a selective fading channel by frequency according to multiple paths through simple calculation of the product of complex numbers in a frequency area. Further, in order to avoid interference within a block and interference between blocks, an OFDM system has a characteristic in that a cyclic prefix (CP), longer than a maximum delay path of a channel, is inserted into the OFDM system.
Having such characteristics, OFDM is currently used as a core modulation scheme in various wireless communication system standards, such as wireless broadband (WiBro), wireless local area network (WLAN), wireless personal area network (WPAN), and 3rd generation partnership project long-term evolution (3GPP LTE).
However, since an OFDM system has a relatively small frequency interval between sub-carriers in comparison with the transmission band thereof and is required to maintain the orthogonality among sub-carriers at the time of transmission, the OFDM system is more sensitive to a frequency offset than a single carrier system. When a frequency offset is generated due to an oscillator mismatch or a Doppler frequency shift between transmitters and receivers, the reception performance may largely deteriorate. Thus, it is required to precisely estimate and compensate for a frequency offset in an OFDM system.
A proposed method for estimating a frequency offset according to the related art includes a scheme for estimation using a differential modulation method in a frequency area, and a scheme using a phase difference using autocorrelation among identical symbols when there are the identical symbols spaced a predetermined distance apart from the frequency area.
There is another method that estimates a frequency offset through decorrelation between a reference signal and a reception signal. A more detailed description thereof will be given below. First, a frequency offset between a reference signal, i.e. a transmission signal, and a reception signal is assumed. Next, a predetermined permutation is extracted from the reference signal in the time area. The predetermined extracted permutation is multiplied by a phase shift value in the time area, which corresponds to the assumed frequency offset, for each sequence index of the permutation. The predetermined permutation of the reference signal, which has been multiplied by the phase shift value, is subjected to a discrete fourier transform (DFT) to be converted into a signal of a frequency area.
Next, a decorrelation signal between the converted frequency area signal and a DFT input of the reception signal is calculated. Here, decorrelation refers to an operation of performing conjugate multiplication on two DFT outputs in a sample-by-sample manner. Conjugate multiplication on two sequences having the length of N is defined by Equation 1 below.
                              ∑                      i            =            0                                N            -            1                          ⁢                                  ⁢                                            x                              i                ⁢                                                                                        ⁡                          (                              y                i                            )                                *                                    Equation        ⁢                                  ⁢        1            
The calculated decorrelation signal is subjected to an inverse DFT (IDFT) again to be converted into a signal in the time area. Finally, a frequency offset value allowing a peak value extracted from a signal of the time area of the decorrelation signal to be at a maximum is obtained while changing the assumed frequency offset value. This method has a large complexity because it requires a process of repeatedly calculating a decorrelation signal to reversely obtain a frequency offset that maximizes the peak value.
The number of times by which a decorrelation signal for the assumed frequency offset is obtained may be determined on the basis of an interval of a permutation used for correlation calculation and an incremental step size configuration. In this case, the calculation of a DFT/IDFT, etc. is required to be performed multiple times, which may increase the complexity and thus make it difficult to implement a reception device.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.